Cannulas are regularly used to introduce and/or extract fluid from a living subject. For example, they are used as part of intravenous feeding and medicament introduction systems in hospitals and the like. They are also used to draw samples of blood for analysis from a patient.
One problem with the cannula structures of the prior art is that they have tended to become uncomfortable to the patient through irritation, particularly when maintained in place for relatively long periods of time. Recently this problem has been alleviated by utilizing a cannula material which swells, and generally softens, on insertion into the patient. Such swellable cannula can be inserted, for example via an over-the-needle technique, by making a relatively small puncture hole in a blood vessel and then withdrawing the needle. Thereafter, the cannula softens and swells providing an increased duct size for introduction and extraction of fluids.
Some such softenable and swellable cannula, for example, those disclosed in U.S Pat. Nos. 4,359,558; 4,424,305; 4,454,309 and 4,439,583, have certain problems of their own. In particular, due to softening the portion of such cannula exterior of the point of puncture of the patient the cannula can become more readily kinked Also, some hydrophilic materials, such as those cited above allow the migration of medicaments through the wall of the cannula and into contact with the puncture wound. This can lead to irritation whereby one of the main advantages of such softenable cannula, namely lack of irritation, can be lost.
It would also be desirable to be able to control the rate of hydration of swellable and/or softenable cannulae so as to assure that they can be fully inserted to a desired distance in the living subject before sufficient hydration occurs to cause enough softening to interfere with insertion of the cannula. This is true in instances wherein the cannula is of significant length and wherein several inches of the cannula may be fed into, for example, a blood vessel to position the end of the cannula in a desired location, for example in or near the heart.
The present invention is directed to overcoming one or more of the problems as set forth above.